Born John Glabas in 1341/42, he became a monk on 1 April 1375. On 25 May 1380 he was named metropolitan of Thessalonica, but left the city in 1382, and was deposed in September 1384. Although reinstated in March 1386, he continued to reside in Constantinople rather than his see until some time between the summer of 1389 and October 1393, when he returned to Thessalonica. During his absence, the city was conquered by the Ottoman Turks in 1387, and he went to negotiations with them in Asia Minor.
- PLP, 4223. Γλαβᾶς ̓Ισίδωρος.